项目结构

entity

1. User

@Data
@NoArgsConstructor
@AllArgsConstructor
@Accessors(chain = true)
public class User implements Serializable {private String username;private String password;private int age;
}

dao

IUserDao

public interface IUserDao {/*** Add user*/int addUser(User user);/*** Get user by id.*/User getUserById(int id);/*** Get all users.*/List<User> getAllUsers();/*** Get users by name.*/List<User> getUsersByName(String name);
}

UserDaoImpl

public class UserDaoImpl implements IUserDao {@Overridepublic int addUser(User user) {System.out.println("Dao: addUser()");return 0;}@Overridepublic User getUserById(int id) {System.out.println("Dao: getUserById()");return null;}@Overridepublic List<User> getAllUsers() {System.out.println("Dao: getAllUsers()");return Collections.emptyList();}@Overridepublic List<User> getUsersByName(String name) {System.out.println("Dao: getUsersByName()");return Collections.emptyList();}
}

service

IUserService

public interface IUserService {void login();void register();
}

UserServiceImpl

public class UserServiceImpl implements IUserService {/*** Each time this interface is called, aenw UserDaoImpl object is created,* resulting in resource waste or causing OutOfMemoryError in serious cases.*/IUserDao userDao = new UserDaoImpl();@Overridepublic void login() {userDao.getAllUsers();System.out.println("This is implementation o login method.");}@Overridepublic void register() {userDao.getAllUsers();System.out.println("This is implementation o register method.");}
}

假设 Web 应用程序现在运行正常。
每次用户调用 IUserService 接口时，都会创建一个 IUserDaoImpl 对象，这会导致资源浪费，在严重情况下可能会引发 OutOfMemoryError。

IUserDao userDao = new UserDaoImpl();

如何解决这个问题❓

我们可以将 IUserDaoImpl 的 Class 作为 key，IUserDaoImpl 实例作为 value 存储在一个HashMap<Class, Object>中。

1. 在 Web 应用程序运行之前，将 ApplicationConext 类中的对象加载并初始化到 IOC 容器中。
2. 当调用 IUserService 接口时，Java 会获取 IUserDaoImpl 对象，而不再需要创建新的 IUserDaoImpl对象。

ApplicationContext

public class ApplicationContext {// Store bean objects in a Hashmap, equivalent to ioc container.private HashMap<Class<?>, Object> beanFactory = new HashMap<>();/*** init ApplicationContext, and put bean objects into IOC container.*/public void initContext() throws IOException, ClassNotFoundException {// Load specified objects to IOC container.beanFactory.put(IUserService.class,new UserServiceImpl());beanFactory.put(IUserDao.class,new UserDaoImpl());}/*** get bean object by class from IOC container.*/public Object getBean(Class<?> clazz) {return beanFactory.get(clazz);}/*** return all bean objects in IOC container.*/public HashMap<Class<?>, Object> getAllBeans() {return beanFactory;}
}

---

配置文件初始化 IOC 容器

❓问题：如上述代码所示，我们可以将指定的对象加载到IOC容器中，但如果我们需要添加一些新对象时，就必须修改源代码，这非常麻烦。

public void initContext() throws IOException, ClassNotFoundException {// Load specified objects to IOC container.beanFactory.put(IUserService.class,new UserServiceImpl());beanFactory.put(IUserDao.class,new UserDaoImpl());// Add new objects into beanFactory.// beanFactory.put(xxxx.class,new xxxxx());// ...........
}

为了解决这个问题，我们可以使用 Java 中的反射。

1. 创建一个application.properties文件，在其中可以添加我们需要的对象信息。

IOC.service.IUserService = IOC.service.Impl.UserServiceImpl
IOC.dao.IUserDao = IOC.dao.Impl.UserDaoImpl

1. 利用反射机制加载对象，并存储到 IOC 容器。

public void initContext() throws IOException, ClassNotFoundException {// Load application.properties file and Get information of bean object.Properties properties = new Properties();properties.load(new FileInputStream("src/main/resources/application.properties"));Set<Object> keys = properties.keySet();for (Object key : keys) {Class<?> keyClass = Class.forName(key.toString());String value = properties.getProperty(key.toString());Class<?> valueClass = Class.forName(value);Object instance = valueClass.newInstance();// put bean object into IOC container.beanFactory.put(keyClass, instance);}
}

RunApplication

public class RunApplication {public static void main(String[] args) throws IOException, ClassNotFoundException {ApplicationContext applicationContext = new ApplicationContext();applicationContext.initContext();// Get all beans.HashMap<Class<?>, Object> allBeans = applicationContext.getAllBeans();Set<Class<?>> classes = allBeans.keySet();for (Class<?> clazz : classes) {System.out.println(clazz.getName() + " -> " + allBeans.get(clazz));}}
}

---

注解初始化 IOC 容器

@Bean

1. 新增 annotation 包，创建 Bean 注解类。

@Target(ElementType.TYPE)    // 只能修饰类型元素
@Retention(RetentionPolicy.RUNTIME)
public @interface Bean {}

2. 在UserDaoImpl 和 UserServiceImpl 类上加上 **@Bean **注解。

@Bean
public class UserServiceImpl implements IUserService {}@Bean
public class UserDaoImpl implements IUserDao {}

3. 在 ApplicationContext 类中添加使用注解初始化 bean 对象。

// root path of project.
private String filePath;/**
* Load all bean objects with annotation @Bean from the project path.
*/
public void initContextByAnnotation() throws ClassNotFoundException, InstantiationException, IllegalAccessException {// Get the project path.filePath = Objects.requireNonNull(ApplicationContext.class.getClassLoader().getResource("")).getFile();// Load all bean objects into IOC container.loadOne(new File(filePath));
}/**
* Load all class file with @Bean.
*/
public void loadOne(File fileParent) throws ClassNotFoundException, InstantiationException, IllegalAccessException {if(!fileParent.isDirectory()){return;}File[] childrenFiles = fileParent.listFiles();if (childrenFiles == null) {return;}for (File child : childrenFiles) {if (child.isDirectory()) {loadOne(child);} else {String pathWithClass = child.getAbsolutePath().substring(filePath.length() - 1);// Get file name like UserServiceImpl.classif (pathWithClass.contains(".class")) {String fullName = pathWithClass.replaceAll("\\\\", ".").replace(".class", "");Class<?> clazz = Class.forName(fullName);if (!clazz.isInterface()) {Bean annotation = clazz.getAnnotation(Bean.class);if (annotation != null) {Object instance = clazz.newInstance();Class<?>[] interfacesList = clazz.getInterfaces();// interface as key, if object has no interface.if (interfacesList.length > 0) {Class<?> interfaceClass = interfacesList[0];beanFactory.put(interfaceClass, instance);}// clazz as key, if object has interface.else {beanFactory.put(clazz, instance);}}}}}}
}

4. 修改 RunApplication 启动类中使用注解初始化 IOC 容器。

public class RunApplication {public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {ApplicationContext applicationContext = new ApplicationContext();//applicationContext.initContext();// Load beans by @Bean.applicationContext.initContextByAnnotation();// Get all beans.HashMap<Class<?>, Object> allBeans = applicationContext.getAllBeans();Set<Class<?>> classes = allBeans.keySet();for (Class<?> clazz : classes) {System.out.println(clazz.getName() + " -> " + allBeans.get(clazz));}}
}

输出：

@Autowired

1. 在 annotation 包中添加 Aurowired 注解类。

@Target(ElementType.FIELD)    // 只能修饰成员变量
@Retention(RetentionPolicy.RUNTIME)
public @interface Autowired {}

2. 在 UserServiceImpl 成员变量添加 @Autowired 注解。

@Bean
public class UserServiceImpl implements IUserService {@Autowiredprivate IUserDao userDao;
}

3. 在 ApplicationContext 类中添加使用注解初始化 bean 对象后，初始化各个 bean 对象的成员变量。
   1. 这里默认只实现了根据类型初始化成员变量（在 springboot 中支持类型和名称）。

/**
* Load all bean objects with annotation @Bean from the project path.
*/
public void initContextByAnnotation() throws ClassNotFoundException, InstantiationException, IllegalAccessException {// Get the project path.filePath = Objects.requireNonNull(ApplicationContext.class.getClassLoader().getResource("")).getFile();// Load all bean objects into IOC container.loadOne(new File(filePath));// initiate fields of object.assembleObject();
}/**
* @Autowired annotation to initiate fields of bean objects.
*/
public void assembleObject() throws IllegalAccessException {Set<Class<?>> keys = beanFactory.keySet();for (Class<?> key : keys) {Object value = beanFactory.get(key);Class<?> clazz = value.getClass();// Set value of field by @Autowired.Field[] declaredFields = clazz.getDeclaredFields();for (Field filed : declaredFields) {Autowired annotation = filed.getAnnotation(Autowired.class);if (annotation != null) {filed.setAccessible(true);// Get bean object by type from IOC container.Object object = beanFactory.get(filed.getType());filed.set(value, object);}}}
}

4. 修改 RunApplication 启动类，并且调用 login() 方法 。

public class RunApplication {public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {ApplicationContext applicationContext = new ApplicationContext();//applicationContext.initContext();applicationContext.initContextByAnnotation();// Get all beans.HashMap<Class<?>, Object> allBeans = applicationContext.getAllBeans();Set<Class<?>> classes = allBeans.keySet();for (Class<?> clazz : classes) {System.out.println(clazz.getName() + " -> " + allBeans.get(clazz));}// Get bean object by class type.IUserService bean = (IUserService) applicationContext.getBean(IUserService.class);// call login method.bean.login();}
}

输出：

Dao 层调用了 getAllUsers() 方法，说明依赖注入成功。

源码：https://github.com/RainbowJier/HandWrittenSpring

Reference

1. 一堂课深挖java反射机制，手撸一个ioc，实现自动装配，顺便spring也了解了_哔哩哔哩_bilibili